A low voltage drop over the voltage regulator is achieved by the use of a MOSFET as a voltage regulating element together with a charge pump providing a sufficiently high gate potential which has to be higher than the output voltage of the voltage regulator, in the case of a low drop regulator even higher than the input voltage of the voltage regulator.
In order to guarantee a substantially constant output voltage, the gate of the voltage regulating MOSFET (e.g. a power MOSFET) is supplied with a bias current provided by a charge pump and controlled by a closed loop control system. That is, the output voltage of the voltage regulator is received by a controller which controls the gate current (and therefore the gate voltage) of the voltage regulating MOSFET such, that the output voltage of the voltage regulator remains substantially constant.
In response to an upward step of the load current (i.e. the output current) the output voltage will slightly drop due to the higher voltage drop over the voltage regulating MOSFET. Triggered by this voltage drop the controller will increase the gate current for charging the gate-source-capacitance of the voltage regulating MOSFET in order to increase the conductivity of the voltage regulating MOSFET thus re-adjusting the output voltage to its desired value.
The time which is needed to compensate for the disturbance in the output voltage induced by the step in a load current is determined by the loop bandwidth of the closed loop control system and especially dependent on the value of the gate-source-capacitance of the voltage regulating MOSFET.
With a given value of the gate-source-capacitance of the voltage regulating MOSFET the speed of the closed loop control system can only be increased by increasing the gate current which charges the gate of the MOSFET. This gate current is supplied by a charge pump, as explained before, and, in order to minimize power consumption, an increase of the maximum gate current which would entail a more costly charge pump is not desirable.